We summarize the experimental program of Jefferson Lab that studies the nucleon spin structure at low Q 2 . This program completes the precise experimental mapping of the nucleon spin structure functions g 1 (ν, Q 2 ) and g 2 (ν, Q 2 ) and their moments started at SLAC, CERN and DESY at high Q 2 , and continued at Jefferson Lab at intermediate Q 2 . The results presented cover the domain where Chiral Effective Field Theory (χEFT) should describe the strong interaction. They provide a

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... ve set of benchmark measurements for χEFT. The preliminary conclusion is that nucleon spin structure data are still challenging for χEFT in spite of the notable improvements in these calculations.